For many years, darkfield scanning methodologies have been used to scan surfaces. Darkfield scanning makes use of light scattered or diffracted by the surface to characterize and examine features of the surface. As used herein, scattered light shall refer to both scattered light and diffracted light. FIG. 1 is a cross-section view of an illuminated surface used to illustrate aspects of darkfield scanning. An illumination source 101 projects a light beam I (also referred to herein as the incident beam) onto the surface 102 being examined. A portion of the incident beam I is reflected by the surface as the reflected beam R. If the surface 102 was perfectly reflective, the entire incident beam I would be reflected. However, most surfaces have a variety of characteristics which cause a portion of the light from an incident beam I to be scattered. Darkfield scanning makes use of this scattered light.
One particular surface feature that causes light scattering is referred to as a defect. The detection, quantification, and classification of defects is important in many areas. In particular, defect detection and analysis are important in semiconductor processing. Defects include, but are not limited to, pits, bumps, scratches, and a number of other features, which mar the surface 102. Thus, the light of an incident beam I is often subject to some degree of scattering. FIG. 1 illustrates a typical incident beam I having a light scattering pattern schematically depicted by a plurality of scattered light rays 103, 104, 105, and 106, which are scattered by a surface defect 108. The depicted plurality of rays can represent a continuous angular distribution of light scattered and diffracted by the surface.
Known darkfield inspection tools use a single discrete photodetector element (for example a PMT) to detect the light scattered from the inspection surface. Some designs use as many as three or four distinct and widely separated discrete photodetector elements. Such discrete photodetector element(s) are positioned so that they are not in the path of the reflected beam R. This results in a detection field where the background (the field) is dark. The scattered light received by the detector provides a representation of the surface 102 whereby the surface defects show up as lighter regions against the dark background or field. Hence, the name darkfield scanning.
FIG. 2 depicts another cross-section view of a surface being scanned using darkfield scanning. The surface 102 is illuminated by an incident beam I, a portion of which is reflected as reflected beam R. Another portion of the light of the incident beam I is scattered. Here, the scattered light is schematically depicted by the rays S1, S2, and S3. Each of the scattered light rays S1, S2, and S3 have scattering angles associated therewith. Because FIG. 2 is a two dimensional representation of a three dimensional reality, only one scattering angle is depicted for each scattered light rays S1, S2, and S3. In the depiction of FIG. 2, the scattering angles are measured from the illuminated surface 102. Thus, scattered light ray S1 is associated with scattering angle A1. Scattered light ray S2 is associated with scattering angle A2. Scattered light ray S3 is associated with scattering angle A3, and so on. The scattering angles S1, S2, S3 depicted here are determined from the surface 102. However, scattering angles can be determined in a variety of different and also in a variety of coordinate systems. For example, the scattering angles can be determined from a line normal to the surface 102.
FIG. 3 is a schematic three-dimensional view of an incident light beam I and a scattered light ray 301. The depicted coordinate system is an (x, y, z) coordinate system with a surface lying in the x-z plane. One scattering angle is depicted as φ, which is the angle from the x-z plane. The other depicted angle is θ, which is the angle from the y-z plane. As was previously stated, many other ways of referring to scattered light ray angles are known and can be used.
One type of conventional darkfield surface inspection device 400 is depicted in FIG. 4. An ellipsoidal mirror 420 is positioned over an inspection surface 402. An incident light beam 401 is directed onto an inspection surface 402. Schematically depicted are a reflected light beam 403 and many scattered light beams 410, 411, 412, 413, 414, 415, and 416. The device includes a first discrete photodetector 421 and a second discrete photodetector 422 positioned above the ellipsoidal mirror 420. A portion of the scattered light (depicted here by scattered light beams 410, 411, 412, 413, 414, 415, and 416) passes through an opening O in the ellipsoidal mirror 420. The center portion of the scattered light beams (schematically depicted by beams 415, 416) passes through a lens 423 which directs the light onto a central mirror 424 which reflects the central beams 415, 416 so they converge at a side focal point 425. The second discrete photodetector 422 is positioned at the side focal point 425 to receive the central beams 415, 416. At the same time, an outer portion of the scattered light beams (schematically depicted by beams 410, 411, 412, 413, 414) passes through the opening in the ellipsoidal mirror 420 and is reflected by the ellipsoidal mirror 420 onto a top focal point 426. The ellipsoidal mirror 420 is specifically designed to concentrate the outer portion of the scattered light beams 410, 411, 412, 413, 414 onto the top focal point 426. Also, the first discrete photodetector 421 is specifically positioned at the top focal point 426. Frequently, the discrete photodetectors 421, 422 include optical feed fibers that convey the focused light to a single discrete photodetector which is commonly a single photodiode or a single photo-multiplier tube (PMT). By integrating light information from the first discrete photodetector 421 and the second discrete photodetector 422 the presence of a defect can be determined.
FIGS. 5(a) and 5(b) are depictions of a portion of a darkfield surface inspection device of the type described in FIG. 4. In FIG. 5(a), a first discrete photodetector 501 is positioned above a focal point 502 (corresponding to another focal point 426 of FIG. 4). Thus, the scattered light is now diverging 503 and out of focus. Large amounts of signal are lost to first discrete photodetector 501 making this a disadvantageous configuration. In FIG. 5(b), a first discrete photodetector 501 is positioned below focal point 502 (also corresponding to another focal point 426 of FIG. 4). Thus, the scattered light 504 is still out of focus and still converging when it reaches the detector 501. Again, large amounts of signal are lost to the first discrete photodetector 501 making this a disadvantageous configuration. This model also applies to the second discrete photodetector (depicted as 422 of FIG. 4). Thus, the prior art systems employ one or more discrete photodetectors to detect scattered light. A disadvantage of such a configuration is that the discrete photodetectors of these implementations are optimized for making measurements of light intensity. Such a configuration can, at best, only collect light over a finite range of angles and is not able to capture any significant amount of spatial information concerning the scattered light
An additional problem with discrete photodetector systems is that, when they are applied to patterned surfaces (e.g., the patterned surfaces of semiconductor wafers), light scattered from the patterned surface is diffracted by the patterned surface in a diffraction pattern. Such a diffraction pattern produces a pattern of light spots a certain discrete angles that are associated with the surface pattern. In an effort to address this problem, conventional approaches use selective filtering to filter out the light spots. In some implementations this is known as “fourier filtering”. An artifact of such fourier filtering is that certain scattering patterns produced by defects will be masked making certain defects difficult to detect. Moreover, as surface patterns become more complicated, as is the case in modern VLSI circuit structures, the scattering patterns become so complicated that it becomes difficult to filter the scattering pattern at all. So, for the most part, it is very difficult to correctly assess whether a scattering pattern is due to defects of normal surface structure. Thus, additional methods and tools must be used to inspect the surface for the presence of defects. This problem slows the inspection process considerably. The cumulative effect of these shortcomings is that more machines and people are required to conduct surface inspection thereby increasing the cost of such inspections.
Thus, conventional inspection systems are very sensitive to photodetector misalignment and variations in individual photdetector characteristics. This problem becomes worse when many individual discrete photodetector elements are employed. As such, alignment difficulties and other related issues put an upper limit on the number of discrete photodetector elements that can be reasonably employed in a given inspection device. Additionally, such conventional devices require the the implementation of filtering for patterned surfaces, are subject to giving false positive readings, and, although they can detect the presence of many defects, they can not determine the type of defects.
For these and other reasons, improved darkfield inspection tools and methodologies are needed.